Could possibly be the mites making the hens very uncomfortable that they wont lay. Also the winter they slow down.
Have you check the coop around dusk and night time. Look at the corners, door window jams, and roosts to see if you can see them buggers moving around.
You can do a complete clean out of the coop with a hose with a nice strong sprayer on it to get into the cracks and corners. Let that dry then spray down with liquid Sevin. Once that is dry dust some of the powdered Sevin on the floors and walls and cover with fresh chips. Repeat in 10 days as it will not kill the eggs.
Don't forget to add some of the Seven dust into their favorite dust bath holes so they can dust themselves with it.
I hope that helps.
Also you can find Sevin Dust and spray at any hardware store or in the garden section of most stores.